FIG. 1 is a network structure of an LTE (Long Term Evolution) system, the related art mobile communication system. For the LTE system, which has evolved from the existing UMTS system, basic standardizations are ongoing in the 3GPP.
An LTE network can be divided into an E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) and a CN (Core Network). The E-UTRAN includes a terminal (or UE (User Equipment)), a base station (eNB (Evolved NodeB), and an access gateway (aGW). The access gateway may be divided into a part that handles processing of user traffic and a part that handles control traffic. In this case, the access gateway part that processes the user traffic and the access gateway part that processes the control traffic may communicate with a new interface. One or more cells may exist in a single eNB. An interface may be used for transmitting user traffic or control traffic between eNBs. The CN may include the access gateway and a node or the like for user registration of the UE. An interface for discriminating the E-UTRAN and the CN may be used.
FIG. 2 shows an exemplary structure of a control plane of a radio interface protocol between the UE and the E-UTRAN based on the 3GPP radio access network standards. FIG. 3 shows an exemplary structure of a user plane of the radio interface protocol between the UE and the E-UTRAN based on the 3GPP radio access network standards.
The structure of the radio interface protocol between the UE and the E-UTRAN will now be described with reference to FIGS. 2 and 3.
The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting data information and a control plane (C-plane) for transmitting control signals. The protocol layers in FIGS. 2 and 3 can be categorized as a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the communication system. The radio protocol layers exist as pairs between the UE and the E-UTRAN and handle a data transmission in a radio interface.
The layers of the radio protocol control plane of FIG. 2 and those of the radio protocol user plane will be described as follows.
The physical layer, the first layer, provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel. Data is transferred between the MAC layer and the physical layer via the transport channel. The transport channel is divided into a dedicated transport channel and a common channel according to whether or not a channel is shared. Between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transferred via the physical channel.
The second layer includes various layers. First, a medium access control (MAC) layer serves to map various logical channels to various transport channels and performs logical channel multiplexing by mapping several logical channels to a single transport channel. The MAC layer is connected an upper layer called a radio link control (RLC) layer by a logical channel. The logical channel is divided into a control channel that transmits information of the control plane and a traffic channel that transmits information of the user plane according to a type of transmitted information.
An RLC (Radio Resource Control) layer, the second layer, segments or concatenates data received from an upper layer to adjust the data size so as for a lower layer to suitably transmit the data to a radio interface. In addition, in order to guarantee various QoSs required by each radio bearer RB, the RLC layer provides three operation modes: a TM (Transparent Mode); a UM (Unacknowledged Mode); and an AM (Acknowledged Mode). In particular, the RLC layer (referred to as an ‘AM RLC layer’, hereinafter) operating in the AM performs a retransmission function through an automatic repeat and request (ARQ) function for a reliable data transmission.
A packet data convergence protocol (PDCP) layer of the second layer performs a function called header compression that reduces the size of a header of an IP packet, which is relatively large and includes unnecessary control information, in order to effectively transmit the IP packet such as an IPv4 or IPv6 in a radio interface having a smaller bandwidth. The header compression increases a transmission efficiency between radio interfaces by allowing the head part of the data to transmit only the essential information.
The RRC layer located at the uppermost portion of the third layer is defined only in the control plane, and controls a logical channel, a transport channel and a physical channel in relation to configuration, reconfiguration, and the release or cancellation of radio bearers (RBs). In this case, the RBs refer to a logical path provided by the first and second layers of the radio protocol for data transmission between the UE and the UTRAN. In general, the set-up (configuration) of the RB refers to the process of stipulating the characteristics of a radio protocol layer and a channel required for providing a particular data service, and setting the respective detailed parameters and operation methods.
The respective radio protocol layers of the LTE are basically based on the radio protocol layers of the UMTS. As described above, the radio protocol layers of the UMTS have the substantially similar functions as those of the LTE. Here, a data processing method of the AM RLC and the PDCP layers, among the second layer, related to the present invention will be described in detail.
FIG. 4 illustrates the processing order in which a transmitting side of AM RLC and PDCP layers of a UMTS receives data from an upper layer, processes the received data, and transmits the processed data;
The order of processing the data received by the transmitting side of the AM RLC and the PDCP layers of the UMTS from the upper layer will now be described with reference to FIG. 4. An SDU (Service Data Unit) refers to data received from an upper layer, and PDU (Protocol Data Unit) refers to data which has been received from an upper layer, processed and then transmitted to a lower layer.
The PDCP layer receives data (PDCP SDU), which is to be transmitted to a lower layer, from an upper layer (S1). The PDCP layer compresses a header of the received data (PDCP SDU) and transfers the same to the lower RLC layer. In this case, a header compressor of the PDCP layer may generate a header-compressed feedback packet by itself irrespective of the PDCP SDU. The header-compressed PDCP SDU or the feedback packet includes PDCP PDUs which are transferred to the lower RLC layer (S2).
When the AM RLC layer receives the RLC SDU, namely, the PDCP PDUs, it segments or concatenates the PDCP PDUs in a fixed size. The AM RLC layer sequentially attaches an RLC sequence number (SN) to the segmented or concatenated data blocks (S4). In this case, the AM RLC layer may generate RLC control PDUs by itself irrespective of the RLC SDU. Here, the RLC SN is not added to the RLC control PDUs. In step S4 as shown in FIG. 4, the RLC PDUs include RLC SN-attached data blocks or RLC SN-free RLC control PDUs. The RLC PDUs are stored in an RLC PDU buffer (S5). This is for a re-transmission of the RLC PDUs that may be necessary later.
When the AM RLC layer transits or re-transmits the RLC PDUs, it performs ciphering by using the RLC PDU SN (S6). In this case, because the ciphering uses the SN, the SN-free RLC PDUs, namely, the RLC control PDUs, are not ciphered. The ciphered RLC PDUs or the non-ciphered RLC control PDUs are sequentially transferred to the lower MAC layer.
In the LTE, the L2 protocol has a room to be improved in various aspects. In particular, the PDCP layer and the AM RLC layer are expected to have the following requirements.
First, in forwarding or re-transmitting unconfirmed PDCP SDUs at handover, the transmitting side forwards or re-transmits only SDUs that have not been received by a receiving side. This is called a selective forwarding or re-transmission.
Second, the size of the RLC PDUs is flexible according to a radio environment at each transmission.
Third, ciphering of the RLC PDU at every transmission or re-transmission is prevented.
These requirements cannot be satisfied by the related art UMTS L2 protocol, so designing of a new L2 protocol is required for the LTE.